Various structures have been developed to supply RF fields to an ionizable gas in a vacuum plasma processing chamber, to excite the gas to a plasma state. The excited plasma interacts with a workpiece in the vacuum plasma processing chamber to etch materials from an exposed workpiece surface or deposit materials on the surface. The workpiece is typically a semiconductor wafer having a planar circular surface, a metal planar surface or a dielectric workpiece, which can have a rectangular periphery, as in a flat panel display.
A processor for treating workpieces with an inductively coupled planar plasma (ICP) is disclosed, inter alia, by Ogle, U.S. Pat. No. 4,948,458, commonly assigned with the present invention. A magnetic field is derived from a coil positioned on or adjacent a single planar dielectric window extending in a direction generally parallel to the workpiece planar surface. In commercial devices, the window is usually quartz because quartz has low material impurity and provides optimum results for RF coupling. The coil is connected to be responsive to an RF source having a frequency in the range of 1 to 100 MHz, but which is typically 13.56 MHz. An impedance matching network is connected between the coil and source, to minimize RF reflections coupled back to the source from a load, including the coil and the plasma.
Barnes et al., U.S. Pat. No. 5,589,737 discloses a plasma processor including a coil for inductively deriving an RF plasma excitation field for processing relatively large substrates, for example, dielectric substrates forming rectangular flat panel displays. In the Barnes et al. patent, the RF field derived by the coil is coupled to the plasma via plural individually supported dielectric windows. In the preferred embodiment of the '737 patent, four such windows are positioned in four different quadrants. To maximize RF coupling from the coil through the windows to the plasma, the windows have a thickness substantially less than the thickness of a single window having the same combined area as the plural windows to withstand the differential pressure between the vacuum inside the chamber and atmospheric pressure on the chamber exterior.
Several different coil configurations are disclosed in the '737 patent. Some of these coils have plural winding segments connected electrically in parallel between first and second terminals coupled to an RF excitation source via a matching network. Some of the coil configurations of the '737 patent have parallel coil segments of the same electrical length between the first and second terminals.
To provide more uniform plasma flux density on the relatively large planar flat panel display surfaces having a rectangular periphery, the various coil configurations disclosed in the '737 patent were redesigned as illustrated in FIG. 1, a bottom view of the redesigned coil. The prior art coil 10 of FIG. 1 includes two spiral-like, electrically parallel copper windings 12 and 14, each having plural spiral-like turns substantially symmetrically arranged with respect to coil center point 16.
Windings 12 and 14 are coplanar and have copper conductors with square cross-sections (with each side having a length of about 1.25 cm), including bottom edges spaced approximately 3 cms above the upper faces of the four rectangular quartz windows 21, 22, 23 and 24, individually supported by one-piece, rigid frame 26, made of a non-magnetic metal, preferably anodized aluminum. Frame 26 is preferably constructed in a manner similar to that illustrated and described in the '737 patent, except that interior mutually perpendicular rails 28 and 30 are substantially coplanar with the top coplanar faces of windows 21-24. Coil 10 is suspended by dielectric hangers from the ceiling of a nonferrous metal (preferably anodized aluminum) electromagnetic shield cover of the type disclosed in Barnes et al. '737.
Windings 12 and 14 respectively include interior terminals 32 and 34, equispaced from coil center point 16 along rail 28. Terminals 32 and 34 are electrically driven in parallel and connected by metal strap 35 and cable 36 to output terminal 38 of matching network 40, having an input terminal connected to be responsive to RF source 42. Typically, strap 35 has an inverted U shape with a first leg of the U being spaced substantially farther from windows 21 and 24 than windings 12 and 14, and the other legs running between the first leg and terminals 32 and 34; strap 35 is shown offset to simplify the drawing.
Windings 12 and 14 also respectively include, at diametrically opposed corners thereof, terminals 44 and 46, respectively connected to ground through capacitors 48 and 50. Output terminal 52 of matching network 40 is also grounded to provide a return current path through capacitors 48 and 50 to the matching network grounded terminal for the parallel currents flowing through windings 12 and 14. Windings 12 and 14 have a geometry and the values of capacitors 48 and 50 are selected so maximum standing wave currents occur along the lengths of windings 12 and 14 at positions that are somewhat electrically close to terminals 44 and 46. Typically, the maximum standing wave currents occur in the outermost turn of each of windings 12 and 14 in proximity to rail 26. The standing wave current is maximized close to the periphery of coil 10 to increase the magnetic flux density at the periphery of the coil and thereby increase the plasma flux density adjacent the workpiece periphery.
Each of windings 12 and 14 has a spiral-like configuration and is long enough that transmission line effects occur therein at the frequency of source 42, as described in the previously mentioned co-pending applications. The configuration of each of the windings 12 and 14 is frequently referred to as a "square or rectangular" spiral. Each of windings 12 and 14 includes 2.125 turns, formed by nine straight segments. Each winding includes four straight metal conducting segments extending parallel to rail 28 and five straight metal conducting segments extending parallel to rail 30, whereby each straight line segment intersects its abutting segment approximately at a right angle. Terminals 32 and 44 of coil 12 are on one side of rail 30 while terminals 34 and 46 of coil 14 are on the opposite side of rail 30. The pitches of the turns of windings 12 and 14 are substantially the same throughout the lengths of the coils between terminals 32, 34 and 44, 46.
The coil of FIG. 1 can be thought of as having center, intermediate and peripheral portions respectively including approximately two, one and two turns. The turns of the center portion include straight metal conducting segments 61-64 of winding 12, as well as straight metal conducting segments 71-74 of winding 14. The one turn of the intermediate portion includes straight segments 75 and 76 of winding 12 as well as straight segments 77 and 78 of winding 14. The turns of the peripheral portion include straight segments 81-83 of winding 12 as well as straight segments 84-86 of winding 14.
The coil illustrated in FIG. 1 has previously been used to excite a plasma for etching rectangular, dielectric flat panel display workpieces having straight, rectangular peripheral sides of 550.times.650 mm and 600.times.720 mm. Such workpieces were fixedly located on an electrostatic chuck so the top face of the substrate was approximately 10 cms from the bottom, interior face of windows 21-24. The rectangular periphery of coil 10 in this prior art arrangement was somewhat greater than the periphery of the rectangular workpiece; in one prior art configuration, coil 10 was dimensioned so the peripheral, straight mutually perpendicular edges thereof were approximately 650.times.750 mm in dimension so the coil extended to the periphery of the rectangular area defined by windows 21-24 and beyond the rectangular periphery of the workpiece.
While the structure illustrated in FIG. 1 functions satisfactorily for certain circumstances, for other circumstances, the uniformity of the plasma flux density across the large area workpieces is not as great as desired. The plasma flux density on the flat panel display workpieces resulting from the coil illustrated in FIG. 1 has a tendency to be relatively low in center and peripheral portions of the workpiece exposed planar rectangular face and to be relatively large in intermediate portions of the workpiece, between the center and peripheral portions thereof. Hence, the plasma flux density on the workpiece has a tendency to be greatest below the coil intermediate portion, i.e., below the second half of the first turn and the first half of the second turn of each of windings 12 and 14 and to be lowest below the center and peripheral portions of coil 10. The decrease in the plasma flux density at the corners of the peripheral regions of the rectangular workpiece is due to a large extent to the tendency of the plasma resulting from the excitation by coil 10 to have a circular periphery. The tendency of the plasma generation region to have a circular periphery results in the plasma flux density in the peripheral regions of the workpiece directly below the peripheral coil portions removed from the coil corners to be substantially greater than the plasma flux density in the peripheral corner regions of the workpiece. The plasma flux density profile along a diagonal of the treated workpiece face deviates approximately 21% from complete uniformity. The relatively low plasma flux density on the substrate surface portions beneath the center and peripheral portions of coil 10 occurs because the plasma flux has a tendency to diffuse from the center of the plasma toward the intermediate portion of the plasma. The coil metal shield structure associated with the vacuum plasma processing chamber has a tendency to cause the magnetic flux derived from coil 10, FIG. 1, to move away from the chamber periphery toward the center of the chamber, as disclosed in the co-pending, commonly assigned Holland et al. application Ser. No. 08/661,203, filed Jun. 10, 1996 now U.S. Pat. No. 5,800,619.
It is, accordingly, an object of the present invention to provide a new and improved vacuum plasma processor for providing a relatively uniform plasma flux density on the surface of a relatively large workpiece.
Another object of the invention is to provide a new and improved vacuum plasma processor having a coil particularly designed so the magnetic flux density derived from it is such that the plasma flux density on a relatively large rectangular workpiece, such as a flat panel display, is relatively uniform, to obviate the tendency for the plasma flux density to be relatively low over center and peripheral regions of the workpiece.
A further object of the invention is to provide a new and improved coil for a vacuum plasma processor, wherein the coil is particularly designed for providing a relatively uniform plasma flux density on the surface of a relatively large workpiece, particularly workpieces having a rectangular periphery.